1. Field of the Invention
This invention relates generally to phase detectors and more particularly to a new and improved analog phase detector comprising digital circuit components and referred to herein as a hybrid phase detector.
2. Description of the Prior Art
Phase detectors which comprise digital components and operate in analog fashion are well known in the present state of the art and are now commonly used for many applications in place of conventional analog phase detectors which are usually considerably more complicated. These hybrid devices are also generally preferred over purely digital phase detector circuits where high accuracy is required at extremely high data rates, for instance, on the order of 25 megacycles or greater. A digital phase detector operating at these frequencies requires a pulse source capable of providing pulses at a frequency at least an order of magnitude greater than the data rate to achieve suitable accuracy. Such pulse sources are not readily available. As will be appreciated, therefore, hybrid phase detectors have been of particular interest in the data processing field for use, for example, in phase locked loops employed in data recording and recovery apparatus.
A phase locked loop typically includes a voltage controlled oscillator controlled to oscillate at a frequency which is an integral multiple of the frequency of the digital data that is to be recovered, for instance, from a magnetic recording medium such as a disc or tape. The voltage controlled oscillator serves to provide a feedback signal which is phase compared with the input digital data signal. Generally, the phase locked loop is designed to operate in a high gain, wide bandwidth mode to achieve rapid lock-on to the input data. Once lock-on is achieved, the loop bandwidth is reduced to avoid response to spurious high frequency signals while maintaining lock-on to long term frequency drift of the input data. For this purpose, it is highly desirable that the phase detector incorporated in the phase locked loop have a linear operating range extending over essentially a full cycle of the clock pulse signals provided by the voltage controlled oscillator.
Another desired operating requirement of a phase locked loop employed in data processing systems is that it be able to maintain lock-on not only for periodic input data as characterized by conventional frequency modulation and phase modulation data but also for aperiodic data which is exemplified by well known codes such as non-return to zero (NRZ) and modified frequency modulation (MFM). Some hybrid phase detectors heretofore available in the art are capable of providing a linear operating range extending over 360.degree. of the voltage controlled clock pulse period but only in response to periodic input data. Others can operate on aperiodic data but have a linear range of only 180.degree., but in any event operation cannot be achieved over 360.degree. in response to both periodic and aperiodic input data.